Radio frequency (RF) receivers have an antenna to pick up RF signals in the air and an RF amplifier having one or more stages, to amplify the RF signals. In order that the RF amplifier have high selectivity to RF signals of the particular frequency to which the receiver is designed to respond, it is provided with resonant circuits which are tuned to such frequency. Then, the receiver will only pick up those signals at such RF frequency and will ignore others. Usually the receiver is provided with an internal antenna, which, because of insufficient sensitivity, is supplemented by an external antenna. However, the simple connection of the external antenna to the receiver causes the resonant circuits in the RF amplifier to become detuned, so as to reduce the gain substantially. Even if the resonant circuits are tuned at the factory for optimum performance with the external antenna connected, the performance has been substantially less than that expected by adding an external antenna.